A compound containing a weakly coordinating anion (i.e., an anion that coordinates only weakly with a cation) is useful in a variety of applications including as an electrolyte and a catalyst. Investigations of very reactive metal and nonmetal cations continues to spur the development of new weakly coordinating anions. See, for example, Bochmann, Angew. Chem., Int. Ed. Engl. 1992, 31 1181; Strauss, Chem. Rev. 1993, 93, 927, Strauss, Chemtracts-Inorganic Chem. 1994, 6,1; and Seppelt, Angew. Chem., Int. Ed. Engl. 1993, 32, 1025. One of the most important uses of weakly coordinating anions is to enhance the catalytic activity of metal cations. Two examples that have received considerable attention recently are metallocene-catalyzed olefin polymerization, and lithium-catalyzed Diels-Alder reactions and 1,4-conjugate addition reactions. See Turner, European Patent Appl. No. 277,004, 1988; Pellecchia et al., Makromol. Chem., Rapid Commun. 1992, 13, 265; DuBay et al., J. Org. Chem. 1994, 59, 6898; Saidi et al., Chem. Ber. 1994, 127, 1761; Kobayashi et al., Chem. Lett. 1995, 307; and Arai et al., Angew. Chem., Int. Ed. Engl. 1996, 15, 3776.
Useful anions must not only be weakly coordinating, they must also be stable with respect to oxidation and/or fragmentation in the presence of highly electrophilic cations. In addition, an ideal weakly coordinating anions should have a single negative charge dispersed over a large surface composed of relatively nonpolar bonds to weakly basic atoms such as hydrogen or the halogens. Weakly coordinating anions which conform to many, but not all, of these criteria include B(Ar.sub.f).sub.4.sup.- (Ar.sub.f =C.sub.6 F.sub.5 or 3,5-C.sub.6 H.sub.3 (CF.sub.3).sub.2), CB.sub.11 H.sub.12-n X.sub.n.sup.- (X=H or I), CB.sub.9 H.sub.10-n X.sub.n.sup.- (X=H, Cl, Br or M(OTeF.sub.5).sub.n.sup.- (n=4, M=B; n=6, M=Nb, Sb)).
All of the anions mentioned above have limitations. Some are too strongly coordinating for specific applications. Some are unstable under the harsh chemical conditions where they would be employed. For example, the fluorinated derivatives of BPh.sub.4.sup.- can react with strongly electrophilic cations, causing (i) cleavage of a C--F bond and formation of a bond between the fluorine atom and the cation or (ii) transfer of a fluoroaryl group to the cation. In either case, the cation is no longer reactive or catalytically active.
Utility of carborane monoanions containing chlorine, bromine and iodine (e.g., CB.sub.11 H.sub.6 Br.sub.6.sup.- and CB.sub.9 H.sub.5 Br.sub.5.sup.-) are limited for several reasons. First, the heavier halogens containing carboranes coordinate more strongly to cations than do fluorine atoms containing carboranes. The order of coordinating ability of HCB.sub.11 H.sub.5 X.sub.6.sup.- ions to the electrophilic Si(i-Pr).sub.3.sup.+ cation is: HCB.sub.11 H.sub.5 Cl.sub.6.sup.- &lt;HCB.sub.11 H.sub.5 Br.sub.6.sup.- &lt;HCB.sub.11 H.sub.5 I.sub.6.sup.-. Therefore, the fluoro derivatives CB.sub.11 H.sub.12-n F.sub.n.sup.- are expected to be less coordinating than any of the known halocarboranes. Another limitation is that CB.sub.11 H.sub.6 Br.sub.6.sup.- and similar anions react with strong reducing agents such as sodium metal (Na). An additional limitation is that carboranes with heavier halogen groups are much easier to oxidize than carboranes containing fluorine groups. For example, HCB.sub.11 H.sub.5 Br.sub.6.sup.- is oxidatively decomposed when treated with elemental fluorine.
The anion CB.sub.11 (Me).sub.12.sup.- is not stable in strong acid and is easily oxidized at only 1.6 V in acetonitrile solution. The most electrophilic cations, i.e., those that require new and more weakly coordinating anions, are extremely oxidizing. Therefore, the oxidation of a weakly coordinating anion at too low oxidation potential is severe limitation in their usefulness.
Most bis(dicarbollide) complex anions have not been halogenated and none have been fluorinated. In addition, they are too strongly coordinating for most applications and are too prone to oxidation. Their stability in the presence of a strong acid is expected to be poor because they are composed of dianions (e.g., C.sub.2 B.sub.9 H.sub.11.sup.2-) surrounding a trivalent M.sup.+3 metal ion. Dianions generally react more readily and more quickly with acids than monoanions.
Other weakly coordinating monoanions, such as CIO.sub.4.sup.-, BF.sub.4.sup.-, PF.sub.6.sup.-, SbF.sub.6.sup.-, Al(OC(Ph)(CF3).sub.2).sub.4.sup.-, Nb(OCH(CF.sub.3).sub.2).sub.6.sup.-, B(OTeF.sub.5).sub.4.sup.-, and Nb(OTeF.sub.5).sub.6.sup.-, are not thermally and/or hydrolytically stable.
Other anions containing boron atoms, and anion containing a carbon atom and a cluster of boron atoms, such as carboranes (e.g., CB.sub.5, CB.sub.9, CB.sub.11), are not particularly stable nor weakly coordinating because the salts formed therefrom contain at most only one fluorine atom which is bonded to a boron atom. Currently, there is no synthetic method which provides carboranes with more than one fluorine atom.
The utility of partially fluorinated carboranes (i.e., carboranes wherein more than one boron atom is fluorinated, but not all boron atoms are fluorinated) such as 7,8,9,10,11,12-CB.sub.11 H.sub.6 F.sub.6.sup.- are commercially limited because other isomers having the formula CB.sub.11 H.sub.6 F.sub.6.sup.- may be concominantly produced in the generation of desired partially fluorinated carboranes, such undesired isomers require costly and time consuming isolation procedures to obtain a substantially pure isomer such as 7,8,9,10,11,12-CB.sub.11 H.sub.6 F.sub.6.sup.-.
Therefore, there is a need for a polyfluorinated carborane anion which is weakly coordinating, and is thermally and hydrolytically stable. There is also a need for a method for producing a salt containing an isomerically enriched or a pure polyfluorinated carborane anion.